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[InstCombine] Preserve signbit semantics of NaN with fold to fabs #136648

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Apr 26, 2025
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[InstCombine] Preserve signbit semantics of NaN with fold to fabs #136648

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98992d3
[InstCombine] Preserve signbit semantics of NaN with fold to fabs
dtcxzyw Apr 22, 2025
fc2ed05
[InstCombine] Use `isKnownNeverNaN`
dtcxzyw Apr 23, 2025
322bb13
[InstCombine] Check nnan on select
dtcxzyw Apr 23, 2025
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6 changes: 5 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2793,7 +2793,11 @@ static Instruction *foldSelectWithFCmpToFabs(SelectInst &SI,

// fold (X <= +/-0.0) ? (0.0 - X) : X to fabs(X), when 'Swap' is false
// fold (X > +/-0.0) ? X : (0.0 - X) to fabs(X), when 'Swap' is true
if (match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X)))) {
// Note: We require "nnan" for this fold because fcmp ignores the signbit
// of NAN, but IEEE-754 specifies the signbit of NAN values with
// fneg/fabs operations.
if (cast<FPMathOperator>(CondVal)->hasNoNaNs() &&
match(TrueVal, m_FSub(m_PosZeroFP(), m_Specific(X)))) {
if (!Swap && (Pred == FCmpInst::FCMP_OLE || Pred == FCmpInst::FCMP_ULE)) {
Value *Fabs = IC.Builder.CreateUnaryIntrinsic(Intrinsic::fabs, X, &SI);
return IC.replaceInstUsesWith(SI, Fabs);
Expand Down
35 changes: 24 additions & 11 deletions llvm/test/Transforms/InstCombine/fabs.ll
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Expand Up @@ -256,6 +256,19 @@ define double @select_fcmp_ole_zero(double %x) {
; CHECK-LABEL: @select_fcmp_ole_zero(
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp nnan ole double %x, 0.0
%negx = fsub double 0.0, %x
%fabs = select i1 %lezero, double %negx, double %x
ret double %fabs
}

define double @select_fcmp_ole_zero_no_nnan(double %x) {
; CHECK-LABEL: @select_fcmp_ole_zero_no_nnan(
; CHECK-NEXT: [[LEZERO:%.*]] = fcmp ole double [[X:%.*]], 0.000000e+00
; CHECK-NEXT: [[NEGX:%.*]] = fsub double 0.000000e+00, [[X]]
; CHECK-NEXT: [[FABS:%.*]] = select i1 [[LEZERO]], double [[NEGX]], double [[X]]
; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp ole double %x, 0.0
%negx = fsub double 0.0, %x
Expand All @@ -268,7 +281,7 @@ define double @select_fcmp_nnan_ole_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp ole double %x, 0.0
%lezero = fcmp nnan ole double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select i1 %lezero, double %negx, double %x
ret double %fabs
Expand All @@ -279,7 +292,7 @@ define double @select_nnan_fcmp_nnan_ole_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp ole double %x, 0.0
%lezero = fcmp nnan ole double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select nnan i1 %lezero, double %negx, double %x
ret double %fabs
Expand All @@ -292,7 +305,7 @@ define double @select_fcmp_nnan_ule_zero(double %x) {
; CHECK-NEXT: [[FABS:%.*]] = call double @llvm.fabs.f64(double [[X:%.*]])
; CHECK-NEXT: ret double [[FABS]]
;
%lezero = fcmp ule double %x, 0.0
%lezero = fcmp nnan ule double %x, 0.0
%negx = fsub nnan double 0.0, %x
%fabs = select i1 %lezero, double %negx, double %x
ret double %fabs
Expand Down Expand Up @@ -320,7 +333,7 @@ define <2 x float> @select_fcmp_nnan_ole_negzero(<2 x float> %x) {
; CHECK-NEXT: [[FABS:%.*]] = call <2 x float> @llvm.fabs.v2f32(<2 x float> [[X:%.*]])
; CHECK-NEXT: ret <2 x float> [[FABS]]
;
%lezero = fcmp ole <2 x float> %x, <float -0.0, float -0.0>
%lezero = fcmp nnan ole <2 x float> %x, <float -0.0, float -0.0>
%negx = fsub nnan <2 x float> <float 0.0, float poison>, %x
%fabs = select <2 x i1> %lezero, <2 x float> %negx, <2 x float> %x
ret <2 x float> %fabs
Expand All @@ -331,7 +344,7 @@ define <2 x float> @select_nnan_fcmp_nnan_ole_negzero(<2 x float> %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan <2 x float> @llvm.fabs.v2f32(<2 x float> [[X:%.*]])
; CHECK-NEXT: ret <2 x float> [[FABS]]
;
%lezero = fcmp ole <2 x float> %x, <float -0.0, float -0.0>
%lezero = fcmp nnan ole <2 x float> %x, <float -0.0, float -0.0>
%negx = fsub nnan <2 x float> <float 0.0, float poison>, %x
%fabs = select nnan <2 x i1> %lezero, <2 x float> %negx, <2 x float> %x
ret <2 x float> %fabs
Expand All @@ -344,7 +357,7 @@ define fp128 @select_fcmp_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
%gtzero = fcmp ogt fp128 %x, zeroinitializer
%gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub fp128 zeroinitializer, %x
%fabs = select i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
Expand Down Expand Up @@ -382,7 +395,7 @@ define fp128 @select_fcmp_nnan_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
%gtzero = fcmp ogt fp128 %x, zeroinitializer
%gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub nnan fp128 zeroinitializer, %x
%fabs = select i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
Expand All @@ -393,7 +406,7 @@ define fp128 @select_nnan_fcmp_nnan_ogt_zero(fp128 %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan fp128 @llvm.fabs.f128(fp128 [[X:%.*]])
; CHECK-NEXT: ret fp128 [[FABS]]
;
%gtzero = fcmp ogt fp128 %x, zeroinitializer
%gtzero = fcmp nnan ogt fp128 %x, zeroinitializer
%negx = fsub nnan fp128 zeroinitializer, %x
%fabs = select nnan i1 %gtzero, fp128 %x, fp128 %negx
ret fp128 %fabs
Expand All @@ -406,7 +419,7 @@ define half @select_fcmp_nnan_ogt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
%gtzero = fcmp ogt half %x, -0.0
%gtzero = fcmp nnan ogt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select i1 %gtzero, half %x, half %negx
ret half %fabs
Expand All @@ -417,7 +430,7 @@ define half @select_nnan_fcmp_nnan_ogt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call nnan half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
%gtzero = fcmp ogt half %x, -0.0
%gtzero = fcmp nnan ogt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select nnan i1 %gtzero, half %x, half %negx
ret half %fabs
Expand All @@ -430,7 +443,7 @@ define half @select_fcmp_nnan_ugt_negzero(half %x) {
; CHECK-NEXT: [[FABS:%.*]] = call half @llvm.fabs.f16(half [[X:%.*]])
; CHECK-NEXT: ret half [[FABS]]
;
%gtzero = fcmp ugt half %x, -0.0
%gtzero = fcmp nnan ugt half %x, -0.0
%negx = fsub nnan half 0.0, %x
%fabs = select i1 %gtzero, half %x, half %negx
ret half %fabs
Expand Down
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